1. Field of the Invention
The present invention relates to an acceleration sensor and a method of manufacturing the same.
2. Description of Related Art
Loading of a sensor (MEMS sensor) employing the MEMS (Micro Electro Mechanical Systems) technique on a portable telephone has recently been started, which has drawn increasing attention to the MEMS sensor. An acceleration sensor for detecting the acceleration of an object is known as a typical MEMS sensor.
FIG. 4 is a sectional view schematically showing the structure of a conventional acceleration sensor.
This acceleration sensor 101 shown in FIG. 4 includes a circuit chip 104 having a circuit for calculating and correcting acceleration, a sensor chip 105 having a piezoresistor (not shown) and a weight 106 of tungsten in a cavity formed by a ceramic package 102 and a shielding plate 103.
The ceramic package 102 has a six-layer structure obtained by laminating six ceramic substrates 102A to 102F, for example. The lower three ceramic substrates 102A, 102B and 102C have rectangular shapes of the same size in plan view. The upper three ceramic substrates 102D, 102E and 102F have the same outline as the ceramic substrates 102A, 102B and 102C in plan view. A rectangular opening is formed on the central portion of each of the ceramic substrates 102D, 102E and 102F. The opening of the ceramic substrate 102D laminated on the ceramic substrate 102C is smaller than that of the ceramic substrate 102E laminated on this ceramic substrate 102D. The opening of the ceramic substrate 102E is smaller than that of the ceramic substrate 102F laminated on this ceramic substrate 102E.
A plurality of pads 107 are arranged on the upper surface of the ceramic substrate 102D. Each of the pads 107 is electrically connected to the circuit chip 104 and the sensor chip 105 through respective bonding wires 108. Wires 109 extending from the pads 107 are formed on the upper surface of the ceramic substrate 102D. The respective wires 109 are connected to an electrode 111 arranged on the lower surface of the lowermost ceramic substrate 102A through via holes 110 vertically penetrate the lower three ceramic substrates 102A, 102B and 102C.
The shielding plate 103 is bonded to the upper surface of the uppermost ceramic substrate 102F so as to close the opening of this ceramic substrate 102F.
The circuit chip 104 is formed of a silicon chip. The circuit chip 105 is bonded to the upper surface of the ceramic substrate 102C through silver paste, with the front surface of a device-forming region thereof facing upward.
The sensor chip 105 is formed by etching a silicon chip from the side of the rear surface (opposite to the front surface on the side of the device forming region) thereof. This sensor chip 105 integrally includes a membrane 112, a frame-shaped support section 113 provided on the peripheral edge portion of the lower surface of the membrane 112, and a weight holding section 114 provided on the central portion of the lower surface of the membrane 112 in the form of a quadrangular prismoid narrowing downward. The membrane 112 is formed by a thin layer portion including the front surface of the silicon chip on the side of the device-forming region. The piezoresistor is built in the membrane 112.
The sensor chip 105 is supported above the circuit chip 104 by chip spacers 115 interposed between the respective corner portions of the support section 113 and the front surface of the circuit chip 104, at a prescribed interval from the front surface of the circuit chip 104.
The weight 106 is fixed to the lower surface of the weight holding section 114 with an adhesive, and arranged between the circuit chip 104 and the sensor chip 105 in a state of not being in contact with the circuit chip 104 and the chip spacers 115.
When acceleration acts on this acceleration sensor 101 and the weight 106 oscillates, the membrane 112 is deformed, and stress acts on the piezoresistor provided on the membrane 112. The resistivity of the piezoresistor changes in proportion to the stress acting thereon. Therefore, the acceleration acting on the acceleration sensor 101 can be determined on the basis of the change in the resistivity of the piezoresistor.
However, the circuit chip 104, the sensor chip 105 and the weight 106 are provided independently of one another, whereby the conventional acceleration sensor is hard to be downsized.